Furnaces or heaters are commonly used to heat fluid, such as air, circulated through a building to heat its interior. Some heaters burn solid fuel, such as wood or coal. The heaters generally include a firebox in which the fuel is burned. Air is circulated to and from the heater via a duct system generally including a cold air duct and a hot air duct communicating with the building interior. The furnace receives air from the building interior via the cold air duct. The air is heated as it flows over the firebox. The heated air is returned to the building interior via the hot air duct to heat the building interior.
Although conventional furnaces of this type work to heat the building interior, the furnaces may suffer from inefficiency in burning the fuel, inefficiency in transferring heat from the products of combustion to the building interior, and high emissions of undesirable combustion by-products. Furnaces are commonly used for many years and can require maintenance and repair for long term durability and desired emissions performance. For example, furnaces with electronic controls can require maintenance to update or replace electronic components. Moreover, in the event of a power outage, the electronic control may become inoperable. Some furnaces may use a catalytic emissions reduction system. Such catalytic systems are prone to blockage and usually do not operate efficiently at low temperatures.